Functional group introduction and aromatic unit variation in a set of π-conjugated macrocycles: revealing the central role of local and global aromaticity

Abstract

π-Conjugated macrocycles are molecules with unique properties that are increasingly exploited for applications and the question of whether they can sustain global aromatic or antiaromatic ring currents is particularly intriguing. However, there are only a small number of experimental studies that investigate how the properties of π-conjugated macrocycles evolve with systematic structural changes. Here, we present such a systematic experimental study of a set of [2.2.2.2]cyclophanetetraenes, all with formally Hückel antiaromatic ground states, and combine it with an in-depth computational analysis. The study reveals the central role of local and global aromaticity for rationalizing the observed optoelectronic properties, ranging from extremely large Stokes shifts of up to 1.6 eV to reversible fourfold reduction, a highly useful feature for charge storage/accumulation applications. A recently developed method for the visualization of chemical shielding tensors (VIST) is applied to provide unique insight into local and global ring currents occurring in different planes along the macrocycle. Conformational changes as a result of the structural variations can further explain some of the observations. The study contributes to the development of structure-property relationships and molecular design guidelines and will help to understand, rationalize, and predict the properties of other π-conjugated macrocycles. It will also assist in the design of macrocycle-based supramolecular elements with defined properties.

Scheme 1. (a) Reversible two-electron reduction and aromaticity switching of the π-conjugated macrocycle [2.2.2.2]paracyclophane-1,9,17,25-tetraene, denoted BCyc here. (b) Visualization of chemical shielding tensors (VIST) with shielded (aromatic) contributions in blue and deshielded (antiaromatic) contributions in red. The VIST plots show that the neutral ground state is dominated by the local aromaticity of the phenylene units; the formally antiaromatic perimeter of [4n] π-electrons only induces weak global antiaromaticity. The doubly reduced state, however, is dominated by the global aromaticity of the resulting [4n + 2] π-electron perimeter. Hydrogen atoms omitted for clarity.

Scheme 2. Systematic structural changes to the π-conjugated macrocycle BCyc to study the evolving properties in a set of [2.2.2.2]cyclophanetetraenes: (i) introduction of electron-withdrawing ester groups and (ii) replacement of the benzene units by naphthalene, anthracene, and pyrene units.

Scheme 3. Synthesis of macrocycles BCyc-Et and BCyc-Hx with benzene units and ethyl/hexyl ester groups. Compound 1a was obtained from 1,4-diiodobenzene (ESI section 1.3.1†).

Scheme 4. (a) Synthesis of macrocycle NCyc-Et with naphthalene units and ethyl ester groups. Compound 1b was obtained from 2,6-dibromonaphthalene (ESI section 1.3.2†). (b) Macrocycles ACyc-Et, PCyc-Et, and PCyc-Hx with anthracene/pyrene units and ethyl/hexyl ester groups, obtained in analogous reactions using precursors 2c–d and 3c–d (ESI sections 1.3–1.6, Fig. S1†).

Fig. 1. UV-vis absorption (solid lines) and photoluminescence spectra (dashed lines) of the macrocycles in CHCl₃ solution (5 μM). The excitation wavelengths for recording the photoluminescence (PL) spectra are shown in brackets; spectra obtained at other excitation wavelengths are available (ESI Fig. S43†).

Fig. 2. Cyclic voltammograms and redox potentials for the reduction of the macrocycles in dichloroethane (DCE), recorded on 2 mm diameter platinum disk electrodes at a scan rate of 0.1 V s⁻¹ with 0.1 M NBu₄PF₆ as the supporting electrolyte. Dotted lines show the measurement of the electrolyte solution without macrocycle. Previously reported redox potential of BCyc in DCE for comparison: −2.29 V vs. ferrocene/ferrocene⁺ (Fc/Fc⁺).

Fig. 3. Calculated redox potentials vs. Fc/Fc⁺ for the reduction to the mono-, di-, tetra- and hexaanions (top) and oxidation to the corresponding cations (bottom) of the macrocycles in 1,2-dichloroethane (DCE). Data available in the ESI Table S2.

Fig. 4. Calculated conformation of (a) ACyc-Et in the neutral and singly charged states and (b) BCyc-Et, NCyc-Et, and PCyc-Et in the neutral state. Panel (a) shows electron density differences between the neutral and charged states (isovalues −0.0015e for ACyc-Et− and +0.0015e for ACyc-Et+). Hydrogen atoms omitted for clarity.

Fig. 5. (a) to (c) VIST plots for BCyc-Et in different charge and spin states. (d) and (e) VIST plots for BCyc for comparison (see also Scheme 1b). Shielded (aromatic) tensor components are shown in blue, deshielded (antiaromatic) tensor components in red. Each tensor component relates to ring currents in a plane perpendicular to it. In all VIST plots, hydrogen atoms were omitted for clarity. Further VIST plots are provided in the ESI section 6.4.

Fig. 6. Comparison of NICS(0)_zz values of the macrocycles in different charge and spin states (singlet states: filled bars, triplet states: unfilled bars). Negative values indicate diatropic (aromatic) macrocyclic currents, positive values indicate paratropic (antiaromatic) macrocyclic currents. The corresponding data is available in the ESI Table S3.

Fig. 7. VIST plots for the dianions of the macrocycles, highlighting that global aromaticity dominates for BCyc-Et2− and NCyc-Et2− whereas local aromaticity remains for ACyc-Et2− and PCyc-Et2−. Hydrogen atoms omitted for clarity.

Fig. 8. Comparison of energetic and magnetic properties of the electronic states of BCyc-Et optimized for the lowest singlet (S₀) and triplet (T₁). Energies relative to the S₀ minimum and the associated absorption (λ_abs) and photoluminescence (λ_PL) wavelengths are shown in the center. VIST plots for the S₀ and T₁ states, each for the S₀ and T₁ geometries, are shown on the sides.